The present invention relates to a configuration of an optical communication system in which optical transmission lines are shared among a plurality of subscriber units, a method for operating the optical communication system, and system expansion such as extension of a transmission distance or increase in the number of accommodated subscribers in the system.
With an increasing demand for communication using broadband, access lines for users have been being shifted to large-capacity access lines using optical fibers in place of access technology based on telephone lines such as DSL (Digital Subscriber Lines). For recent access line services, PON (Passive Optical Network) systems (hereinafter also referred to as PONs simply or as optical passive network systems or passive optical network systems) are often used in view from the line construction cost and the maintenance management cost. Standardization in ITU-T (International Telecommunication Union Telecommunication Standardization Sector) is a typical example (see ITU-T Recommendations G984.1, G984.2 and G984.3). Introduction of GPON (Gigabit PON) into access networks have begun globally since about 2006.
PON is a system in which optical signals split and multiplexed using optical fibers and an optical splitter are transmitted and received between a station-side apparatus (hereinafter referred to as “OLT (Optical Line Terminal)”) and subscriber units (hereinafter referred to as ONUs (Optical Network Units)). Due to limitation caused by the attenuation of an optical signal passing through each optical fiber or the number of optical branches in the optical splitter, there is a certain critical distance for communication between the OLT and each ONU. To take a specific example, a GPON is used with communication sections set at up to 20 km and with the number of branches (the number of ONUs which can be connected to an OLT) set at up to 64.
With increasing opportunities for ordinary home subscribers (communication network users) to gain access to the Internet for communication to gather information or to organize their social lives, it has been requested to organize communication networks, particularly to enhance access network providing services for connecting the subscribers to the communication networks. That is, any carrier providing a communication network is driven under the pressure of capital expansion for increasing the number of users who can be accommodated in each station with increase of the number of users in each access line. As a method for increasing the number of users, it can be considered to introduce an additional PON for use in an access network, that is, to add another OLT, or to expand the number of users who can be accommodated. i.e. the number of ONUs which can be accommodated, in each PON system. In a typical PON, however, an OLT carries out complicated system control including the bandwidth control and the administration of ONUs accommodated therein. The OLT is much more expensive than each ONU. In addition, the cost to lay new optical fibers brings large expenses to the carrier. From the above description, expansion of the number of ONUs which can be accommodated in each OLT is a desirable solution.
On the other hand, in order to enhance access network providing services, a next-generation PON which is referred to as 10GPON (10 Gigabit PON) or 10GEPON (10 Gigabit Ethernet PON) has been looked into in ITU-T or IEEE (Institute of Electrical and Electronics Engineers) as means for transmission at a higher bit rate than the background art. In transmission at such a higher bit rate, attenuation or variance of optical signals passing through optical fibers has greater influence than in transmission at a bit rate according to the background art. Accordingly, in order to build a system with a communication distance as long as that in an existing PON, it is necessary to provide optical receiving devices with wider dynamic ranges, optical fibers with higher performance, and a variance compensation function. Although higher bit rates may increase the number of users who can be accommodated, increase in the development cost will become a problem.